1. Field of the Invention
The present invention relates generally to an apparatus and a method for channel estimation in a mobile communication system and more particularly, to an apparatus and a method for estimating Channel Impulse Response (CIR) for a location where a power of a channel exists so as to enhance performance of a channel estimator.
2. Description of the Related Art
Advances in the communication industry and an increase in user demand for Internet service has recently increased the desirability for communication systems that can efficiently provide Internet service. To respond to this demand, a Broadband Wireless Access (BWA) system has been introduced with enough broadband to meet the increasing user demand for an efficiently provided Internet service.
The BWA system integrally supports voice services and multimedia application services such as various low-speed and high-speed data services and high-definition video. The BWA system accesses a Public Switched Telephone Network (PSTN), a Public Switched Data Network (PSDN), the Internet, an International Mobile Telecommunications (IMT)-2000 network, and an Asynchronous Transfer Mode (ATM) network in a mobile or stationary environment based on radio media using broadbands of 2 GHz, 5 GHz, 26 GHz, and 60 GHz, and supports a channel transfer rate over 2 Megabits per second (Mbps). A BWA system can be classified as a broadband wireless subscriber network, a broadband mobile access network, and a high-speed wireless Local Area Network (LAN) based on the terminal mobility (stationary or mobile), the communication environment (indoor or outdoor), and/or the channel transfer rate.
The radio access scheme of the BWA system is standardized by Institute of Electrical and Electronics Engineers (IEEE) 802.16 Working Group, which is an international standardization organization.
Compared to a conventional radio technique for the voice service, the IEEE 802.16 standard enables the transfer of more data within a shorter time over a wide data bandwidth and enables all users to efficiently share and utilize channels (or resources). Also, with Quality of Service (QoS) guaranteed, the users can enjoy services of different qualities according to the service characteristics.
The IEEE 802.16 communication system adopts Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme for physical channels. That is, using the OFDM/OFDMA scheme, the BWA system achieves high-rate data transmission by sending physical channel signals using a plurality of subcarriers.
The BWA system supports mobility of a Mobile Station (MS) using a multi-cell structure, wherein every cell utilizes the same frequency for better efficiency. However, in this multi-cell based system, interference from the neighboring cells greatly affects its performance.
A transmitter in the wireless access system encodes information data to transmit and then generates a pilot signal. The transmitter then allocates the data symbols and the pilot symbols to the subcarriers and converts the symbols to a time-domain signal through Inverse Fast Fourier Transform (IFFT).
Accordingly, a receiver eliminates a guard interval from the received signal, converts the received signal to a frequency-domain signal through FFT, estimates the channel using the pilot signal, and equalizes a single tap channel using the estimated channel. The receiver uses the received, channel-equalized signal to determine a Log Likelihood Ratio (LLR) and generates final information bits by channel-decoding using the LLR.
A channel estimator of the receiver estimates the channel at the pilot position, performs the conversion to the time domain using Inverse Discrete Fourier Transform (IDFT), estimates a maximum time delay of the channel using the converted Channel Impulse Response (CIR), extracts only an effective CIR by applying a time window suitable for a time delay, and converts to the frequency domain by applying a DFT to the extracted effective CIR.
Disadvantageously, when a rate of a guard band is not negligible relative to a used band, channel estimation drastically deteriorates at a band edge due to spectral leakage.
Further, because a time window is applied using only a maximum time delay of the channel, the CIR includes considerable noise power with a considerable dispersion in the time axis. Thus, in a region having a low Signal to Noise Ratio (SNR), performance is degraded.
To address these shortcomings, a method for applying a virtual pilot and a frequency domain window has been suggested. This method helps diminish a band-edge performance degradation to some degree, but is still subject to the performance degradation caused by an inaccuracy of a virtual pilot generation and a channel decoding performance degradation caused by noise coloring of a frequency window.
In this regard, a successive interference cancellation method for searching a path of the maximum power to the time domain, removing distortion components using the searched path, and then searching a maximum path was suggested. However, when the accuracy of an initial estimation value falls, which is a fundamental problem of successive interference cancellation, the interference cannot be effectively eliminated.